U.S. Pat. No. 3,650,708 discloses a process for preplating substrates, particularly plastics, by subjecting the substrate to phosphorus sesquisulfide in an organic solvent to deposit phosphorus sesquisulfide at the surface of the substrate and thereafter subjecting the best treated surface to a metal salt or complex thereof to form a metal-phosphorus-sulfur compound. The thus treated surface is either conductive or is capable of catalyzing a reduction of a metal salt to produce a conductive surface. Such conductive surfaces are readily electroplated by conventional techniques.
This process has been successfully applied to many plastics such as polypropylene, ABS, polyvinyl chloride, polyethylene, polybutene, polyurethane, and other well known plastics. A more complete list of typical plastics to which the above process can be applied is included below. In addition, this process may also be used on other non-metal substrates, examples of which are also listed below.
Although this process is operable on all of the plastics described above and in more detail below, certain industrial and commercial uses of plastic articles having a metal plate, such as in the automobile industry, require a metal plate which is smooth, uniform, and substantially free of cracks, bubbles, and other irregularities. For example, ABS can be readily plated by first preplating it according to the process listed above, and subsequently electroplating it by conventional processes. However, for commercial applications, it would be desirable to improve the adhesion of the metal plate to the ABS surface for the intended commercial use.
When ABS is preplated by conventional processes which do not use phosphorus sesquifide, the surface of the ABS which is to be preplated must be etched before it can be subjected to the conventional process. Etching the surface of the ABS substrate comprises subjecting the substrate to various acidic solutions to produce varied small holes, pores and channels in the surface. Thus, etching is a destructive process in which the surface of the substrate is partially eroded by the etching solution.
Although etching is a common preliminary step in conventional preplating processes, it is not required in preplating processes using elemental phosphorus or low oxidation number phosphorus compounds such as P.sub.4 S.sub.3, which are disclosed in U.S. Pat. Nos. 3,556,956; 3,650,708; and 3,666,637. In those processes the substrates are first subjected to a solvent for the purpose of swelling the surface of the substrate.
The use of organic glycol preconditioners in the conventional CrO.sub.3 preplating process is well known. For example, U.S. Pat. No. 3,817,774 describes some conventional processes using the organic glycols as preconditioners in the treatment of ABS. However, because the mechanism of the phosphorus and phosphorus compound preplating processes differ from the mechanisms of the conventional preplating process, it is not totally predictable that treatment steps for conventional processes will function well when applied to the phosphorus related processes described above.